The emergence of extreme ultraviolet (EUV) projection lithography has placed stringent demands on interferometric metrology systems. In order to achieve diffraction-limited performance, EUV lithographic systems require wavefront tolerances on the order of 0.02 waves rms (0.3 nm rms at a wavelength of 13.4 nm)..sup.1 While the accuracy of interferometry is typically limited by the quality of the reference surface or wave, a class of interferometers has been developed in which extremely high quality reference waves are created by diffraction from small apertures..sup.2-5
EUV lithographic systems rely on wavelength-specific reflective multilayer coatings. To accurately probe phase effects in these resonant reflective structures, at-wavelength metrology is required. Various at-wavelength interferometric measurement techniques including lateral-shearing interferometry,.sup.6 Foucault and Ronchi testing.sup.7 have been reported. These methods, however, have yet to demonstrate the accuracy required for the development of EUV lithographic imaging systems. In order to meet the accuracy challenge, an EUV-compatible diffraction-class interferometer, the phase-shifting point diffraction interferometer (PS/PDI), was developed by Medecki et al..sup.8,21
The PS/PDI is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. In the PS/PDI, as illustrated in FIG. 1A, the optical system 2 under test is illuminated by a spherical wave 5 that is generated by an entrance pinhole 6 in a mask 4 that is placed in the object plane of the optical system 2. To assure the quality of the spherical-wave illumination, pinhole 6 is chosen to be several times smaller than the resolution limit of the optical system. Grating 8 splits the illuminating beam 5 to create the required test and reference beams 10 and 12, respectively. A PS/PDI mask 20 is placed in the image plane of the optical system 2 to block the unwanted diffracted orders generated by the grating 8 and to spatially filter the reference beam 12 using a reference pinhole 16. The test beam 10, which contains the aberrations imparted by the optical system, is largely undisturbed by the image-plane mask by virtue of it passing through window 14 in the PS/PDI mask 20 that is large relative to the point-spread function (PSF) of the optical system under test. The test and reference beams propagate to the mixing plane where they overlap to create an interference pattern recorded on a CCD detector 18. The recorded interferogram yields information on the deviation of the test beam from the nominally spherical reference beam. FIG. 1B depicts a PS/PDI mask 21 comprising a square shaped window 22 and a reference pinhole 24 to the right which has a diameter of less than 100 nm.sup.8, 10, 11, 21.
The original PS/PDI.sup.8 requires the image-plane beam separation to be sufficient to prevent the reference beam from passing through the test-beam window. For a given separation, this requirement places limits on the scattering and aberrations that can be present in the optic under test. If these limits are not met, the accuracy of the PS/PDI is compromised. It is not feasible to simply increase the image-point separation to strictly meet the above separation requirements due to the unreasonable fringe density this would produce.
The reference wavefront in the PS/PDI has been shown to have a systematic-error-limited accuracy.sup.9 of 0.05 nm, and the PS/PDI has been successfully used in testing prototype EUV lithographic systems..sup.10,11 One of the drawbacks of the PS/PDI, however, is its susceptibility to scatter or high-frequency features that can cause confusion of the test and the scattered-reference beams. In the presence of this scattered light, the reference beam is no longer a clean spherical wave, but includes high-frequency features that make the interferometry more difficult. Mid-spatial-frequency features in the wavefront of interest are especially susceptible to this problem.